In previous studies. regional variations in the expression of the Na+-Ca2exchanger (NCX) have been examined qualitatively in human heart using the C
2C12 monoclonal antibody [Wang, J., Schwinger, R.H., Frank, K., Muller-Ehms
en, J., Martin-Vasallo, P., Pressley, T.A., Xiang, A., Erdmann, E. & McDono
ugh, A.A. (1996) J. Clin. invest. 98. 1650-1658]. Although NCX expression w
as found to be significantly lower in the atria compared to the septum, no
significant differences were found between atrial and ventricular tissue. N
CX has been located in the general sarcolemma and t-tubules of ventricular
muscle and as t-tubules are sparse in atrial tissue compared to ventricular
tissue, it is surprising that NCX expression was found to be similar in bo
th atria and ventricles [Wang et al. (1996)]. To reinvestigate this, we hav
e used SDS/PAGE and a quantitative Western blotting technique to determine
the pattern of expression of NCX in guinea-pig heart in tissue samples from
left atrium, right atrium, septum, left ventricle and right ventricle. NCX
protein expression was 17.5 +/- 3.9 pmol.mg(-1) of protein in the left atr
ium and 29.2 +/- 6.1 pmol.mg(-1) of protein in the right atrium, which were
both significantly lower (P < 0.05) than NCX expression in the septum, lef
t ventricle and right ventricle (64.7 +/- 15.2, 76.8 +/- 19.5 and 69.4 +/-
14.1 pmol.mg(-1) of protein, respectively, n = 7). These differences in NCX
expression may reflect variations in the cellular location of NCX protein
in these regions. To study this, we used confocal immunofluorescence of sin
gle isolated myocytes to examine differences in the proportion of fluoresce
nt staining on the general surface membrane compared with the interior of t
he cell (which presumably reflects a t-tubular location). We found that the
general membrane staining was 79.0 +/- 1.2% in cells from the atr ia which
was significantly higher (P < 0.001) than that seen in cells from the sept
um, left ventricle and right ventricle, with 48.1 +/- 1.1%, 48.2 +/- 1.8% a
nd 45.6 +/- 1.3%, respectively (n = 20). These results illustrate a similar
pattern of NCX expression in guinea-pig and human, with expression in atri
al tissue significantly lower than in ventricular tissue. However, the cell
ular location of NCX differs regionally; in atrial tissue, the majority of
the NCX protein is located in the general sarcolemma whereas in ventricular
and septal tissue, approximately 50% of NCX protein is located within the
cell (presumably at the level of the t-tubules).